l1vpn WG Hamid Ould-Brahim
Don Fedyk
Internet Draft Nortel
Expiration Date: May 2007
Yakov Rekhter
Juniper Networks
October 2006
BGP-based Auto-Discovery for L1VPNs
draft-ietf-l1vpn-bgp-auto-discovery-01.txt
Status of this Memo
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Abstract
The purpose of this draft is to define a BGP-based auto-
discovery mechanism for layer-1 VPNs. The auto-discovery
mechanism for l1vpns allows the provider network devices to
dynamically discover the set of PEs having ports attached to
CEs member of the same VPN. That information is necessary for
completing the signaling phase. One main objective of l1vpn
auto-discovery mechanism is to support "single-end
provisioning" model, where addition of a new port to a given
l1vpn would involve configuration changes only on the PE that
has this port and on the CE that is connected to the PE via
this port.
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draft-ietf-l1vpn-bgp-auto-discovery-01.txt October 2006
Changes to previous version:
- Added section 4 on carrying TE attribute in BGP.
- Added section 5 on scalability for using BGP as an auto-
discovery mechanism for l1vpns.
- Completed section 6 on Security Considerations.
- Added section 7 on IANA considerations.
1. Introduction
The purpose of this draft is to define a BGP-based auto-
discovery mechanism for layer-1 VPNs. The auto-discovery
mechanism for l1vpns allows the provider network devices to
dynamically discover the set of PEs having ports attached to
CEs member of the same VPN. That information is necessary for
completing the signaling phase. One main objective of l1vpn
auto-discovery mechanism is to support "single-end
provisioning" model, where addition of a new port to a given
l1vpn would involve configuration changes only on the PE that
has this port and on the CE that is connected to the PE via
this port.
The auto-discovery mechanism proceeds by having a PE advertises
to other PEs, at a minimum, its own IP address and the list of
<private address, provider address> tuples local to that PE.
Once that information is received, the remote PEs will identify
the list of VPN members they have in common with the
advertising PE, and use the information carried within the
discovery mechanism to perform address resolution during
signaling phase.
PE PE
+---------+ +--------------+
+--------+ | +------+| | +----------+ | +--------+
| VPN-A | | |VPN-A || | | VPN-A | | | VPN-A |
| CE1 |--| |PIT || BGP route | | PIT | |-| CE2 |
+--------+ | | ||<----------->| | | | +--------+
| +------+| Distribution| +----------+ |
| | | |
+--------+ | +------+| | +----------+ | +--------+
| VPN-B | | |VPN-B || -------- | | VPN-B | | | VPN-B |
| CE1 |--| |PIT ||-( GMPLS )--| | PIT | |-| CE2 |
+--------+ | | || (Backbone ) | | | | +--------+
| +------+| --------- | +----------+ |
| | | |
+--------+ | +-----+ | | +----------+ | +--------+
| VPN-C | | |VPN-C| | | | VPN-C | | | VPN-C |
| CE1 |--| |PIT | | | | PIT | |-| CE2 |
+--------+ | | | | | | | | +--------+
| +-----+ | | +----------+ |
+---------+ +--------------+
Figure 1 BGP auto-discovery for l1vpn
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This version of the draft focuses on describing an auto-
discovery mechanism for the basic mode only. Details for the
enhanced mode will be described in future revised version of
this draft.
2. Procedures
In the context of l1vpns, a CE is connected to a PE via one or
more ports, where each port may consists of one or more
channels or sub-channels. Each port on a CE that connects the
CE to a PE has an identifier that is unique within that l1vpn
(but need not be unique across several l1vpn). We refer to this
identifier as the customer port identifier (CPI). Each port on
a PE has as well an identifier that is unique within that
provider network. We refer to this identifier as the provider
port identifier (PPI). Note that IP addresses used for CPIs,
PPIs could be either IPv4 or IPv6 addresses.
A PE maintains for each l1vpn configured on that PE a port
information tables (PIT) associated with each l1vpn that has at
least one port configured on a PE. A PIT contains a list of
<CPI, PPI> tuples for all the ports within its l1vpn. Note that
a PIT may as well hold routing information (for example when
CPIs are learnt using a routing protocol).
A PIT on a given PE is populated from two sources: the
information related to the CEs ports attached to the ports on
that PE (this information could be optionally received from the
CEs), and the information received from other PEs through the
auto-discovery mechanism. Well refer to the former as the
"local" information, and to the latter as the "remote"
information.
Propagation of local information to other PEs is accomplished
by using BGP multiprotocol extensions as specified in [BGP-VPN-
AUTODISCOVERY]. To restrict the flow of this information to
only the PITs within a given l1vpn, we use BGP route filtering
based on the Route Target Extended Community [BGP-COMM], as
follows.
Each PIT on a PE is configured with one or more Route Target
Communities, called "export Route Targets", that are used for
tagging the local information when it is exported into
providers BGP. The granularity of such tagging could be as
fine as a single <CPI, PPI> pair. In addition, each PIT on a PE
is configured with one or more Route Target Communities, called
"import Route Targets", that restrict the set of routes that
could be imported from providers BGP into the PIT to only the
routes that have at least of these Communities.
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When a service provider adds a new l1vpn port to a particular
PE, this port is associated at provisioning time with a PIT on
that PE, and this PIT is associated (again at provisioning
time) with that l1vpn.
Note that since the protocol used to populate a PIT with remote
information is BGP, since BGP works across multiple routing
domains, it follows that the mechanisms described in this
document could support l1vpns that span multiple routing
domains.
3. Carrying l1vpn information in BGP
The <CPI, PPI> mapping is carried using the Multiprotocol
Extensions BGP [RFC2858]. [RFC2858] defines the format of two
BGP attributes, MP_REACH_NLRI and MP_UNREACH_NLRI that can be
used to announce and withdraw the announcement of reachability
information. We introduce a new a new subsequent address family
identifier (to be assigned by the IANA), and also a new NLRI
format for carrying the CPI and PPI information.
One or more <PPI, CPI> tuples could be carried in the above
mentioned BGP attributes.
The format of encoding a single <PPI, CPI> tuple is shown in
Figure 2 below:
+---------------------------------------+
| Length (1 octet) |
+---------------------------------------+
| PPI Length (1 octet) |
+---------------------------------------+
| PPI (variable) |
+---------------------------------------+
| CPI AFI (2 octets) |
+---------------------------------------+
| CPI (length) |
+---------------------------------------+
| CPI (variable) |
+---------------------------------------+
Figure 2: NLRI BGP encoding
The use and meaning of these fields are as follows:
Length:
A one octet field whose value indicates the length of
the <PPI, CPI> Information tuple in octets.
PPI Length:
A one octet field whose value indicates the length of
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of the PPI field
PPI field:
A variable length field that contains the value of
the PPI (either an address or <port index,
address> tuple
CPI AFI field:
A two octets field whose value indicates address
family of the CPI.
CPI Length:
A once octet field whose value indicates the
length of the CPI field.
CPI (variable):
A variable length field that contains the CPI
value (either an address or <port index, address>
tuple.
If the value of the Length of the Next Hop field is 4, then the
Next Hop contains an IPv4 address. If this value is 16, then
the Next Hop contains an IPv6 address.
4. Carrying L1VPN Traffic Engineering Information in BGP
In addition to reachability information, the auto-discovery
mechanism may carry Traffic Engineering information that will be
used for signaling purposes. For example a PE may learn
from the remote PEs, the switching capability, the maximum LSP
bandwidth of the remote l1vpn interfaces. This document proposes
the use of the BGP attribute defined in [BGP-TE-ATTRIBUTE] to
carry such information.
5. Scalability
Recall that the Service Provider network consists of (a) PE, (b)
BGP Route Reflectors, (c) P nodes (which are neither PEs nor
Route Reflectors), and, in the case of multi-provider VPNs, (d)
ASBRs.
A PE router, unless it is a Route Reflector should not retain
L1VPN-related information unless it has at least one VPN with an
Import Target identical to one of the VPN-related information
Route Target attributes. Inbound filtering should be used to
cause such information to be discarded. If a new Import Target
is later added to one of the PE's VPNs (a "VPN Join" operation),
it must then acquire the VPN-related information it may
previously have discarded.
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This can be done using the refresh mechanism described in [BGP-
RFSH]. The outbound route filtering mechanism of [BGP-ORF],
[BGP-CONS] can also be used to advantage to make the filtering
more dynamic.
Similarly, if a particular Import Target is no longer present in
any of a PE's VPNs (as a result of one or more "VPN Prune"
operations), the PE may discard all VPN-related information
which, as a result, no longer have any of the PE's VPN's Import
Targets as one of their Route Target Attributes.
Note that VPN Join and Prune operations are non-disruptive, and
do not require any BGP connections to be brought down, as long
as the refresh mechanism of [BGP-RFSH] is used.
As a result of these distribution rules, no one PE ever needs to
maintain all routes for all L1VPNs; this is an important
scalability consideration.
Route reflectors can be partitioned among VPNs so that each
partition carries routes for only a subset of the L1VPNs
supported by the Service Provider. Thus no single route
reflector is required to maintain VPN-related information for
all VPNs.
For inter-provider VPNs, if multi-hop EBGP is used, then the
ASBRs need not maintain and distribute VPN-related information
at all. P routers do not maintain any VPN-related information.
As a result, no single component within the Service Provider
network has to maintain all the VPN-related information for all
the VPNs. So the total capacity of the network to support
increasing numbers of VPNs is not limited by the capacity of any
individual component.
An important consideration to remember is that one may have any
number of INDEPENDENT BGP systems carrying VPN-related
information. This is unlike the case of the Internet, where the
Internet BGP system must carry all the Internet routes. Thus one
significant (but perhaps subtle) distinction between the use of
BGP for the Internet routing and the use of BGP for distributing
VPN-related information, as described in this document is that
the former is not amenable to partition, while the latter is.
6. Security Considerations
This document describes a BGP-based auto-discovery mechanism
which enables a PE that attaches to a particular L1VPN to
discover the set of other PE routers that attach to the same
VPN. Each PE router that is attached to a given VPN uses BGP
to advertise that fact. Other PE routers which attach to the
same VPN receive these BGP advertisements. This allows that set
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of PE to discover each other. Note that a PE will not always
receive these advertisements directly from the remote PEs; the
advertisements may be received from "intermediate" BGP
speakers.
It is of critical importance that a particular PE should not be
"discovered" to be attached to a particular VPN unless that PE
really is attached to that VPN, and indeed is properly
authorized to be attached to that VPN. If any arbitrary node
on the Internet could start sending these BGP advertisements,
and if those advertisements were able to reach the PE nodes,
and if the PE nodes accepted those advertisements, then anyone
could add any site to any L1VPN. Thus the auto-discovery
procedures described here presuppose that a particular PE
trusts its BGP peers to be who they appear to be, and further
that it can trusts those peers to be properly securing their
local attachments. (That is, a PE must trust that its peers
are attached to, and are authorized to be attached to, the
L1VPNs to which they claim to be attached.).
If a particular remote PE is a BGP peer of the local PE, then
the BGP authentication procedures of RFC 2385 can be used to
ensure that the remote PE is who it claims to be, i.e., that it
is a PE that is trusted.
If a particular remote PE is not a BGP peer of the local PE,
then the information it is advertising is being distributed to
the local PE through a chain of BGP speakers. The local PE
must trust that its peers only accept information from peers
that they trust in turn, and this trust relation must be
transitive. BGP does not provide a way to determine that any
particular piece of received information originated from a BGP
speaker that was authorized to advertise that particular piece
of information. Hence the procedures of this document should
be used only in environments where adequate trust relationships
exist among the BGP speakers.
7. IANA Considerations
SAFI number to be assigned by IANA for carrying l1vpn
information in the NLRI.
8. References
[BGP-VPN-AUTODISCOVERY] Ould-Brahim, H., Rosen, E., Rekhter,
Y., "Using BGP as an Auto-Discovery Mechanism for Layer-3
and Layer-2 VPNs", draft-ietf-l3vpn-bgpvpn-auto-05.txt,
work in progress
[BGP-TE-ATTRIBUTE] Ould-Brahim, H., Fedyk, D., Rekhter, Y.,
"Traffic Engineering Attribute",
draft-fedyk-bgp-te-attribute-01.txt, work in progress.
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[GVPN] Ould-Brahim, H., Rekhter, Y., et al., "Generalized VPNs
using BGP and GMPLS toolkit", work in progress, August 2005.
[BGP-COMM] Ramachandra, Tappan, et al., "BGP Extended
Communities Attribute", draft-ietf-idr-bgp-ext-communities-
08.txt, August 2005, work in progress.
[BGP-MP] Bates, Chandra, Katz, and Rekhter, "Multiprotocol
Extensions for BGP4", February 1998, RFC 2283.
[BGP-RFSH] Chen, A., "Route Refresh Capability for BGP-4", RFC
2918, October 2000.
[BGP-ORF] Chen, E., and Rekhter, Y., "Outbound Route
Filtering Capability for BGP-4", draft-ietf-idr-route-
filter-16.txt, Work in Progress.
[BGP-CONS] Marques, P., et al., "Constrained VPN route
distribution", draft-ietf-l3vpn-rt-constrain-02.txt,
work in progress.
[L1VPN-FRMK] Tomonori Takeda, et al., "Framework and
Requirements for Layer 1 Virtual Private Networks", draft-
ietf-l1vpn-framework-04.txt, October 2006, work in
progress.
9. Author's Addresses
Hamid Ould-Brahim
Nortel
P O Box 3511 Station C
Ottawa ON K1Y 4H7 Canada
Phone: +1 (613) 765 3418
Email: hbrahim@nortel.com
Yakov Rekhter
Juniper Networks
1194 N. Mathilda Avenue
Sunnyvale, CA 94089
Email: yakov@juniper.net
Don Fedyk
Nortel
600 Technology Park
Billerica, Massachusetts
01821 U.S.A
Phone: +1 (978) 288 3041
Email: dwfedyk@nortel.com
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